The present invention relates to a tropospheric scatter communication system, and more particularly, to a tropospheric scatter communication system having an angle diversity response capability.
Tropospheric communication has emerged from its uncertain beginnings in the early nineteen fifties to become a robust communication system that fills a gap between line-of-sight microwave links and long range HF or LF links. Tropospheric forward scatter occurs in the region between the stratosphere and the earth's surface in the presence of "blobs" of atmosphere having refractive index variations. Such variations are the result of differences in temperature, pressure and gaseous constituents, the main variable being water vapor. When irradiated by microwave or UHF signals the blobs re-radiate the signals in all directions, some of which scatter in the forward direction to produce electromagnetic fields at the receiving location.
The collection and analysis of empirical data from experimental and operational tropospheric scatter sites characterize its statistical performance in terms of short term and long term amplitude-time distributions. Short term distributions, measured over intervals of tens of seconds, describe a Rayleigh distribution, from which hourly median values are obtained. Long term distributions represent the variation of the hourly median values over longer periods of time, a month a season or year, and vary considerably with the season of the year and with geographical location.
Methods of coping with short term (Rayleigh) fading have been devised through the use of "diversity" transmission paths; paths that are independent and therefore afford greater reliability than a single transmitter/receiver at each end. Effectively proven methods employ space diversity, (two or more antennas spaced approximately 200 wavelengths apart), frequency diversity, (two or more carriers separated in the MHz), polarization diversity, and time diversity (repetition of the same information when slow data rates are involved).
The use of any one method may be utilized independently in dual diversity, or may be combined judiciously in quadruple diversity. In many operational systems, space and frequency diversity are combined to provide reliable communication links. However, since two widely separated antennas are required for space diversity, this necessitates a large communications site. Furthermore, substantial costs are involved in the construction of the remote second antenna, its mounting base, and the transmission of signals thereto.